Method for determining sulfate concentration

ABSTRACT

Method for determining the sulfate ion concentration of an aqueous solution. The mechanism of the method resides in the use of a chelate of a metal which under acidic conditions dissociates to release metal ions which will react with the sulfate ion present to produce a colloidal metal sulfate precipitate. In order to ensure that the free chelant does not precipitate or react to form a precipitate, there is also added to the solution a quantity of a metal compound, the metal of which will replace the original metal of the chelate to form a water soluble second chelate. The turbidity or color density of the aqueous solution is measured and compared to the color density values obtained for solutions containing known quantities of sulfate precipitate of the same metal.

United States Facet [22] Filed [45] Patented [73] Assignee Dec. 29, 1969Nov. 23, 197i Betz Laboratories, Inc. Trevose, Pa.

[54] METHOD FOR DETERMINING SULFATE CONCENTRATEON 8 Claims, No Drawings[52] US. Cl 23/230 R G0ln 31/02,

G01n 31/22 [50] Field at Search 23/230 [56] References Cited OTHERREFERENCES Sporek; K. F Anal. Chem. 30, No. 6. June. 1958. 1,032- 1.035.

Primary Examiner-Morris O. Wolk Assistant Examiner-R. M. ReeseAttorneys-William J. Holcomb and Alexander D. Ricci ABSTRACT: Method fordetermining the sulfate ion concentration of an aqueous solution. Themechanism of the method resides in the use of a chelate of a metal whichunder acidic conditions dissociates to release metal ions which willreact with the sulfate ion present to produce a colloidal metal sulfateprecipitate. in order to ensure that the free chelant does notprecipitate or react to form a precipitate, there is also added to thesolution a quantity of a metal compound, the metal of which will replacethe original metal of the chelate to form a water soluble secondchelate. The turbidity or color density of the aqueous solution ismeasured and compared to the color density values obtained for solutionscontaining known quantities of sulfate precipitate of the same metal.

METHOD FOR DETNING SULFATE CONCENTRATION BACKGROUND OF THE INVENTION Inmany situations there is the need to ascertain the sulfate ionconcentration of an aqueous solution without utilizing the well-knowntechniques such as the gravimetric procedures which are time consumingand require delicate techniques and sophisticated equipment.

Oftentimes where sulfate determinations are required, the economics aresuch that manpower, time and equipment are prohibitive. Such situationsare in the water treatment industry where tests are constantly beingconducted in order to insure proper operational levels. In this fieldmany control tests are constantly being conducted on a work shift basisin many cases and accordingly the time and equipment which can bedevoted to each individual test is limited.

Sulfate ion concentrations and control thereof in waters of industrialboiler systems and cooling water systems are important since the sulfateion concentration gives an indication as to the condition of the water.For example in cooling water sulfate ion determinations are conducted toinsure that the concentrations of such do not exceed the solubilitypoint under the particular operational conditions which if exceeded willproduce scale due to precipitation and deposition of calcium sulfate. Inboiler water, sulfate ion concentrations are important in order toestablish in conjunction with conductivity the dissolved solids of thewater are accordingly the rate of blowdown necessary to avoidsaturation, concentration and finally precipitation and deposition ofthe original constituents which were previously in a dissolved form.

Since the goal of any test procedure is minimum time and manpower tieupand minimum equipment expenditure together with consistent accuracy, theindustry is constantly looking for newer, faster and more accurate testmethods and of course, not least of all simple tests which can beexplained to an individual worker in a mater of minutes with theexpectation that he will understand and be capable of conducting thetest in a short precipitate Until the advent of the present invention aprocedure was utilized which generally called for the precipitation ofthe sulfate ion of a solution by the addition thereto of to 30 meshbarium chloride crystals. Since that system as with the presentinvention used turbidity or color density as the frame of reference formeasuring the sulfate content, it was imperative that the bariumchloride crystals dissolve slowly and uniformly to pennit the formationof colloidal barium sulfate particles. The use of larger crystal sizesresulted in nonuniform dissolution and accordingly large flocs ofprecipitate which had the tendency to occlude the aqueous phase andsettle. As can be appreciated the turbidity measurements in this casewere not accurate and were not reproducible.

Because the procedure was completely dependent upon the particular meshsize to insure proper and uniform dissolution and consequently uniformprecipitation, care need always be observed to assure that theindividual crystal particles would not aggregate due to presence of aminor amount of moisture. The system was too delicate for an industrywhich requires simple, quick and accurate tests. Moreover bariumchloride crystals of the size required are no longer commerciallyavailable. Accordingly it was applicants goal to devise a quantitativetest for sulfate which although simple, quick and accurate did notsuffer from the peculiarities and the sensitivities of the methoddescribed above.

GENERAL DESCRIPTION OF THE INVENTION The method of the present inventiongenerally comprises adding to a sample of a sulfate containing aqueoussolution, a water soluble metal chelate which possesses the property ofdissociating under acid conditions to release ions of the metal and freechelant. With proper pH control i.e. adjusting from generally theneutral state to the acidic state slowly, and preferably to a pH of fromabout 2.5 to 5.5, the metal ions are released slowly and uniformly so asto permit reaction of the metal ion with the sulfate ions present toproduce the colloidal precipitate. Generally there is some idea as tothe relative range of the sulfate concentration present in the solution,therefore it is necessary of course that sufficient metal chelate beadded to insure that total reaction of the sulfate ions occurs.

In order to assure that the free chelant does not react in such a way asto produce a competitive or obscuring precipitate, a water solublecompound of second metal is added to the sample solution. This compoundin solution of course yields ions of the metal. The metal ions, however,must be of such a nature that reaction with the free chelant ispreferential and the resultant chelate is water-soluble. In this mannerit is assured that the colloidal precipitate formed is only the metalsulfate.

Although it is not necessary a stabilizing material can be added eitherbefore, during or after the pH adjustment. The primary function of thestabilizing material is to maintain the colloidal metal sulfateprecipitate dispersed in the aqueous solution to thereby provide anaccurate turbidity or color density reading.

As is evident from the foregoing since the test is based upon turbidityor color density readings, the volume of sample and the volume of theaddition of the particular reagent should be controlled in order toassure a proper relationship with a control. In this regard it isadvisable to utilize a smaller volume sample of the solution to betested that the reference solution, since distilled water can be addedto increase the volume of test sample to conform with that of thereference solution. At this point it should perhaps be explained thatthe reference solution is prepared with a known quantity of sulfate ionand the solution is treated in accordance with the present invention tothereby permit the obtention of photometric readings for turbidity orcolor density.

Standard sulfate solutions are prepared in accordance with ASTMSTANDARDS" Part 23, 1968 pp. 54-62. These standard solutions areprepared so as to obtain in accordance with the above method readingsfor the inspection of turbidities or color densities for solutionscontaining 0, 5, l0, I5, 20, 25, etc. parts by weight of sulfate ion permillion parts of weight of solution, Accordingly when turbidity or colordensity readings for a sample solution are taken, these reading may becompared with the reference solutions to thereby accurately determinethe concentration of the sulfate ion. Although the method has beenproven to be accurate by testing under many circumstances, there arecertain precautions that should be observed. For example since the testis predicated upon a measurement of light transmission, any turbidsample of solution should be filtered and refiltered if necessary, toobviate any increase in the ultimate readings. As can be appreciated thepresence of undesirable colloidal matter will ultimately result inhigher and inaccurate reading.

In addition, care must be taken and the procedure should be modified inthe determination of sulfate concentrations below5 p.p.m. This problemarises because the solubility of barium sulfate which although low willat these levels provide a sizable percentage error in the ultimatereadings. This problem however can be overcome by either concentratingthe sample or by adding, for example 5 milliliters of a standard sulfatesolution (I ml.=0. mg. sulfate) to a 50 ml. sample. This will add 0.5mg. of sulfate to the sample which must be subtracted from the finalresult.

Having thus described the invention broadly, a specific embodiment isset forth as illustrative of the invention and is not to be consideredlimitative thereof.

PREPARATION OF REAGENTS Reagent No. l

Weigh on a torsion balance 50.0 grams of sodium chloride and 26.5 gramsof aluminum chloride. Dissolve the salts in approximately 500 ml. ofdistilled water. By means of the graduated cylinder transfer 20 ml. ofthe concentrated hydrochloric acid to the solution. Dilute the solutionwith distilled water to 750 ml. Pour 250 ml. of the glycerine reagentinto the solution and mix well. Reagent No. 2

50.0 grams of the barium chloride and 80.0 grams of the ethylene diaminetetraacetic acid-di-sodium salt into approximately 900 ml. of distilledwater. Adjust the pH of the solution to 7.0 using solutions ofhydrochloric acid and/or sodium hydroxide. Dilute the solution to 1liter with distilled water.

PRGCEDURE Filter the sample if turbid and adjust the sample to near roomtemperature (15 to 30 C.). Pipet into the 100 ml. beaker 50 ml. of theclear sample and add by pipet 10 ml. of Reagent No. 1. Stir the solutionuntil mixed and pour it into the optical cell. Set the instrument tozero absorbance. Pour the sample solution back into the beaker and addby pipet 5 ml. of Reagent No. 2. Set the timer immediately and stir thesolution for 5 seconds. Let stand for 4 minutes and pour the solutioninto the optical cell. Measure the turbidity at the 5 minute mark.

Using the above described procedure and reagents: test solutionscontaining known quantities of sulfate ions were produced according tothe ASTM STANDARDS," Part 23, 1968, pp. 54-62.

The test solution contained the parts per million of sulfate as setforth in the following table. Duplicate solutions for each concentrationwere produced. One set of the test solutions were subjected to theprocedure outlined above to obtain electrically a reading for theturbidity. These results were recorded. The duplicate samples were thentreated in accordance with the procedure to establish whether theabsorbancc values obtained closely approximated those of the initialtest series. From the data recorded in the following table, it wasapparent that the values obtained coincided within reasonable limitswith those recorded for the initial test series and that the procedureis in fact accurate for the purpose claimed.

TABLE Absorbence readings X 1,000 Duplicate sam- Inltlal sample lorpiesolutions Solutions with the with the specified sulfate InitialDuplicate specified sulfate concentrations samples samples concentrationAlthough the foregoing explanation has a dealt exclusively with the useof barium-ethylene diamine tetraacetic acid as the barium chelatecompound it is obvious that other chelating agents such asbarium-nitrilotriacetic acid (or salt) chelate compound or a bariumdiethylenetriamine pentaacetic acid chelate compound can be used.Likewise, although aluminum chloride has been specified as the watersoluble compound of the second metal, various water salts of copper,iron. trivalent chromium, zinc, etc., can also serve the same purpose.it should be appreciated that since sulfate is being measured thesulfate of the metal of course will not be used. Such salts as copperchloride, iron chloride (ferric), chromous chloride and zinc chloridehave been used successfully. The general prerequisites for the metalcompound is that the metal at one which will readily react with the freechelant of the barium chelate and which will not react with the sulfatepresent to form a precipitate.

in addition hydrochloric acid has been named as the acid used to adjustthe pH, however as is obvious, other acids other than sulfuric, ofcourse, can be used for this purpose so long as the acid does notinterfere with the mechanism of the procedure. Similarly glycerin hasbeen stated as the stabilizing agent. However any compound or materialcan be used which performs the same function with the prerequisite, ofcourse that it does not possess an appreciable adsorbence value of itsown so as to add to the turbidity readings.

in order to ascertain whether in fact other ions or components in awater sample would interfere with the test, various samples of waterwere taken from different locations within the United States and fromvarious installations where water was being used industrially for thepurpose set forth in table 2. The samples as received were tested forsulfate concentration according to the procedure outline above. Afterthe sulfate concentration for was determined for of the samples, 20parts per million were added to additional portions of the respectivesamples and each sample was again subjected to the procedure outlinedabove. The sequence of course was utilized to ascertain the sulfateconcentration of the sample as received. This known value then permitsan accurate assay when the samples bearing the added sulfate ion weresubjected to the test procedure. The results of the evaluation were asfollows.

Sump of cooling tower.

The results of the test recorded in the above table confirmedconclusively that the test method operated effectively with water frommany sources and that no interference by additional ions and componentsin the water was observed. it should also be noted, of course, that thewater used in both cooling water towers and as boiler feed has generallybeen treated not only with corrosion inhibitors such as phosphates,chromates and zinc but also dispersants such as natural and syntheticpolymers, organic phosphonates, etc. Accordingly the present methodoperated effectively even with these materials or ions present.

Having thus described my invention, what is claimed is:

l. A method for detennining the sulfate ion concentration of an aqueoussolution which comprises mixing said solution with (i) a quantity ofwater-soluble metal chelate compound which under acidic conditions willdissociate to release the quantity of metals ions necessary to reactwith all of the sulfate ions in said solution to form colloidalprecipitate, (ii) a water soluble compound of a second metal which underacidic conditions will provide metal ions which will react to replacethe metal of the metal chelate compound to form a second watersolublechelate; adjusting the aqueous solution to an acid pH whereby saidmetals ion from said metal chelate compound are released and react withsaid sulfate ions of said solution to produce the colloidal precipitateand said ions of said second metal reacts so as to replace said metal ofthe metal chelate to thereby produce the water-soluble chelate of saidsecond metal and measuring the sulfate concentration by comparing theturbidity or color density of said solution with those of standardsolutions containing known quantities of colloidal metal sulfateprecipitate.

2. A method according to claim 1 wherein a material capable ofstabilizing the precipitate formed upon adjustment of the solution to anacid pH is added to said solution.

3. A method according to claim 1 wherein said metal chelate compound isa barium-ethylenediaminetetraacetic acid chelate and said water-solublecompound of said second metal is aluminum chloride.

4. A method according to claim 3 wherein the pH of said solution isadjusted to a pH of from about 2.5 to about 5.5 by the addition ofhydrochloric acid.

5. A method according to claim 4 wherein a material capable ofstabilizing the precipitate formed upon adjustment of the solution to anacidic pH is added to said solution.

6. A method according to claim 5 wherein said pH is adjusted by adding asolution containing hydrochloric acid, sodium chloride and glycerine.

7. A method according to claim 3 wherein said aqueous solutioncontaining said sulfate is filtered prior to the addition of saidchelate compound and said second metal compound.

8. A method according to claim 1 wherein the metal chelate compound isselected from the group consisting of bariumethylenediaminetetraaceticacid chelate, bariumnitrilotriacetic acid chelate andbarium-diethylene-triamine pentaacetic acid chelate, and saidwater-soluble compound of a second metal is selected from the groupconsisting of copper chloride, ferric chloride, chromous chloride andzinc chloride.

2. A method according to claim 1 wherein a materIal capable ofstabilizing the precipitate formed upon adjustment of the solution to anacid pH is added to said solution.
 3. A method according to claim 1wherein said metal chelate compound is abarium-ethylenediaminetetraacetic acid chelate and said water-solublecompound of said second metal is aluminum chloride.
 4. A methodaccording to claim 3 wherein the pH of said solution is adjusted to a pHof from about 2.5 to about 5.5 by the addition of hydrochloric acid. 5.A method according to claim 4 wherein a material capable of stabilizingthe precipitate formed upon adjustment of the solution to an acidic pHis added to said solution.
 6. A method according to claim 5 wherein saidpH is adjusted by adding a solution containing hydrochloric acid, sodiumchloride and glycerine.
 7. A method according to claim 3 wherein saidaqueous solution containing said sulfate is filtered prior to theaddition of said chelate compound and said second metal compound.
 8. Amethod according to claim 1 wherein the metal chelate compound isselected from the group consisting of barium-ethylenediaminetetraaceticacid chelate, barium-nitrilotriacetic acid chelate andbarium-diethylene-triamine pentaacetic acid chelate, and saidwater-soluble compound of a second metal is selected from the groupconsisting of copper chloride, ferric chloride, chromous chloride andzinc chloride.